Sci-fi games recommendation

Deus Ex games, Prey, Chronicles of Riddick, Singularity, Timeshift, ...I enjoyed all of them ;)

There's Tribes Ascend, but I'm not sure if it's not mainstream, been a long time I stopped playing.

Well, it's F2P though.

Homeworld

Metroid, Earthbound? To the Moon has some sci-fi elements...

Section 8 and Section 8 Prejudice

Aliens versus Predator from 2010 was actually quite enjoyable :)

System Shock 2. It's coming to steam "soon" supposedly, but it's currently available on GOG.

1. Chaser;
2. Binary Domain;
3. E.Y.E: Divine Cybermancy;
4. MDK 2;
5. Messiah;
6. Space Rangers 2;
7. Freelancer;
8. X-Tension series (X, X-2 and X-3);
9. Red Faction;
10. Beyond Good & Evil;
11. Oni;
12. Rogue Trooper;
13. Aliens vs Predator (Aliens vs Predator 2 (also Add-On Primal Hunt));
14. SiN (and SiN Episodes - Emergence);
15. Star Trek: Voyager - Elite Force (Star Trek: Elite Force 2);
16. Star Wars: Knights of the Old Republic 1-2;
17. The Thing;
18. Bionic Commando;
19. Bioshock 1-2;
20. Blade Runner.

Ok then some less popular games:

• Giants Citizen Kabuto
• Project Nomads
• Tron 2.0

• Anachronox
• Tron 2.0

Fallout, Fallout 2, Fallout 3, Deus Ex, Deus Ex Human Revolution, Bioshock, Bioshock 2, Arcanum

FTL!! ;D

Beneath a Steel Sky
Gemini Rue
The Longest Journey
Resonance
Space Quest
Waking Mars
Xenosaga

Not yet mentioned:

Sins of A Solar Empire

Supreme Commander

Chrome + Chrome Spec Force
Deus Ex series
Serious Sam series

AVP 2010 :D

Oh boy, here have some suggestions:

E.Y.E. Divine Cybermancy - Do you know what's happening in this game? No. But that's okay, cause you'll like it anyway.
FTL - BUY THIS NOW, RIGHT NOW (while its on sale, and get it from the developers as it gives you a drm-free copy for you humble library and a steam key)
Waking Mars - A great game.
Primordia - Scifi point and click adventure. I hear good things.
S.P.A.Z. - Fun, although gets to be a bit of a slog halfway through. On sale right now though for the Linux sale.
Miner Wars 2081 - Descent 2012.
Retrovirus - Descent 2013.
Beyond Good & Evil - An older action/adventure game, very good.
System Shock 2 - Now on gog!
Gimbal - A great top-down space shooter on greenlight
Fallout: New Vegas - Maybe not the kind of sci-fi you're looking for, and definitely mainstream, but if you haven't played this you should rectify that...
Natural Selection 2 - Alien v. Human multiplayer game.
Command & Conquer 3 - Mainstay of RTS.
Endless Space - Space 4X Strategy.
Sins of a Solar Empire - More Space Strategy!
AI War: Fleet Command - EVEN MORE SPACE STRATEGY.

Also, these aren't out yet but you might be interested in them:

Planetary Annihilation - A Kickstarted RTS game that will involve mutliple planets and galaxies as the battlefields. From the makers of Total Annhiliation.
Starbound - From one of the makers of Terraria, a similar pixely game, but in space and with a ton of content and procedurally generated planets.
Star Command - A Star Trek-like isometric build and fight spaceships game.
Routine - FPS-stlye moonbase horror. Greenlit, should be out soon.
Kinetic Void - Also greenlit but awaiting release, this is a 3D space fighter/simulator with a focus on complete spaceship customization.

Hope you find something :)

aliens clonial marines:D

Tribes: Ascend

Test Metro 2033 the new come soon , bioshok 1,2 same the 3 is soon .
Borderland 1,2 Fallout.

FPS: Elite Force 2. Skip the first, n2 is just more of it in every aspect. :)
Adventure: Star Trek 25th anniversary. <- free
Puzzle-exploration: Waking Mars.

I'd say Omikron and Anachronox. Especially Anachronox. If you're into point and click though, go for Gemini Rue, or if you're a hardcore fan of the genre try and hunt down The Ward (won't be easy...) or download 6 Days a Skeptic (hope that's the right title), or pick up The Dig on Steam (bit of a pixel hunt there though).